Pneumatic tires containing air are mounted on wheels of a bicycle or a car. Even if air has been pumped into such pneumatic tires until the air pressure therein reaches a prescribed value, the air gradually escapes therefrom and the air pressure therein decreases with a lapse of time. A large decrease in tire pressure causes problems, such as making steering difficult. Thus, when the air pressure in pneumatic tires has decreased to a value which is much lower than a prescribed value, air must be supplied thereinto.
However, when the air pressure in a pneumatic tire is lower than a prescribed value, it is difficult to recognize it from the appearance thereof. Thus, in some cases, the pneumatic tire is used as it is, which might cause a danger. Also, it is troublesome to supply air into a pneumatic tire with an air supply pump or the like every time the air pressure therein decreases.